Poly(phenylene sulfide) (hereinafter often abbreviated to “PPS”) fibers have high heat resistance, chemical resistance, electrical insulating properties, and flame retardancy and hence, have come to be used in industrial material applications including various filters, electrical insulators, and papermaking canvases. In particular, PPS fibers are extensively used in filter cloths for use in various industrial filters, e.g., bag filters for dust collection from discharge gases.
In use as bag filters, the discharge gases from incineration facilities have high temperatures and the filter cloths are used over a span of several years as a life. Namely, in industrial material applications such as filter cloths for bag filters, excellent long-term heat resistance is important, and changing little in fiber structure and decreasing little in strength at high temperatures of 150-210° C. are important properties.
It is generally known that an improvement in the degree of crystallization is effective in inhibiting the strength from decreasing at high temperatures and in improving dimensional stability, and various methods of heightening the degree of crystallization have been proposed. Examples thereof include a method in which a heightened spinning speed is used to enhance stretching in the spinning part, thereby controlling the degree of crystallization to be in a specific range (Japanese Patent No. 4852104) and a method in which a heat treatment is conducted at a temperature of 120-280° C. for a period of several seconds to several minutes to thereby improve the degree of crystallization (Japanese Patent No. 5139998 and JP 2013-72148 A). However, the fibers obtained by either of these methods cannot have a sufficiently high degree of crystallization, and the stability of the fiber structure thereof is insufficient.
Examples of techniques of obtaining PPS fibers having a high degree of crystallization include a method in which a blend of PPS with another thermoplastic resin is spun to obtain a composite fiber, this composite fiber is then stretched and heat-treated, and the other thermoplastic resin is thereafter dissolved away to obtain a nanofiber (WO 2013/125514). When this method is used, there is a possibility that the degree of crystallization can be heightened, but it is difficult to diminish movable amorphous components, which serve to perform molecular motion. The fiber obtained hence has insufficient structural stability. Furthermore, not only is the fiber usable in limited applications since the fiber is a nanofiber, but also the step of dissolving away the other thermoplastic resin is necessary.
Meanwhile, a PPS fiber which has a narrow molecular-weight distribution and a low content of metallic impurities and is hence excellent in terms of suitability for fiber formation and fiber property has been disclosed (JP 2008-202164 A). However, there is no mention therein of any technique for enhancing the high-temperature stability of the fiber structure in using the PPS fiber and thereby inhibiting the strength from decreasing.
It could therefore be helpful to provide a PPS fiber which, when used at high temperatures, changes little in fiber structure such as the degree of crystallization, and decreases little in strength and, hence, has excellent long-term heat resistance and is suitable for use as bag filters.